The ALPINE sample allows for the first time a statistical analysis of the merger fraction shortly after the Epoch of Reionization. The galaxies are robustly categorized in mergers and nonmergers (e.g., rotators or dispersion-dominated systems) using the full ALMA C\(^+\)  cube in position–velocity space via a morpho-kinematic approach similar to studies of H\(\alpha \)  at lower redshifts (e.g., [1]}, [2]}) and by fitting a tilted ring model (see also [3]}, [4]}). The classification procedures of both methods are detailed in [5]} and [6]}. Of the 75 C\(^+\) -detected ALPINE galaxies, [7]} identified 23 of them as major merger systems in the mass range of \(\log (M/{M}_{\odot })\)  = 9–11 with a mass ratio less than 4, a velocity difference between the components of less than \(500\,{\rm km/s}\) , and a physical distance of less than \(20\,{\rm kpc}\) . The authors computed the fraction of major mergers in this sample (\(\sim \)\(45\%\)  and \(\sim \)\(35\%\)  for \(z\)\(\sim \)\(4.5\)  and \(z\)\(\sim \)\(5.5\) , respectively). These values are a factor of two higher than at \(z\)  = 2–3 (\(\sim \)\(20\%\) ) and other studies at the same redshift using only HST imaging (e.g., [8]}). The latter could highlight an incompleteness due to missing dust-obscured merger components in the UV and optical data. The computed fraction of major mergers is consistent with that predicted by the Evolution and Assembly of Galaxies and their Environments (EAGLE) hydrodynamical simulation [9]}.
The major merger fraction of 35–45% derived from the ALPINE sample by [7]} suggests that major mergers could play a significant role in galaxy evolution at these redshifts. To estimate the contribution to the galaxy mass assembly, a merger timescale has to be assumed, which comes with significant uncertainty. Assuming typical merger timescale representations by [11]}, [12]}, and [13]}, the contribution of major mergers to the SFR density at \(z=\)  4–6 varies from less than \(5\%\)  (for Kitzbichler et al.) to nearly \(30\%\)  (for Snyder et al.).
